Many sophisticated navigation systems rely upon receipt of a sequence of electromagnetic wave signals, such as radio wave signals, from signal sources that may be tens of miles to thousands of miles from the receiver. The signals may be reflected or otherwise affected and thus arrive at the receiver as two or more multipath signals displaced in time from each other. Ideally, the receiver will either severely discriminate against all but one of these replicas; or the receiver will equalize and/or compensate for the time delays and shape distortions present in the composite signal.
Several workers have proposed approaches for reducing or removing multipath signal errors. Lakatos, in U.S. Pat. No. 3,537,008, distinguishes between a direct or "specular" path from transmitter to receiver and one or more "echo" paths that provide multipath signals for that transmission. A modulated pilot signal is transmitted together with a message signal, and arrival of the known modulated signal is continuously monitored at the receiver. The modulated signal is of finite length and acts as a timing signal to open and close a gate at the receiver. The contemporaneously transmitted direct signal is admitted through the gate, but later arriving echo signals are not admitted. This system uses two parallel signal processing paths at the receiver, one path containing a phase locked loop for recognition of the modulated signal. If an echo signal arrives before the receiver gate closes, a portion of the echo signal will also be admitted and treated as part of the direct message signal.
Decomposition of the arriving signal into selected contiguous but nonoverlapping frequency intervals to reduce multipath signal error is disclosed in U.S. Pat. No. 3,542,954, issued to Flanagan. The transmitted direct signal and any echo signals are received and normalized at a plurality of spaced apart transducers, at each of which this frequency decomposition occurs. Among the transducers, the maximum signal in each frequency interval is chosen, and a composite direct signal at the intended receiver is constructed from this maximum signal set.
U.S. Pat. No. 3,566,035, issued to Noll et al, discloses use of the "complex cepstrum" (Fourier transform of the logarithm of the power spectrum plus phase angle) of short time interval, partly overlapping segments of a transmitted signal to determine periodicity or aperiodicity of a received signal. Flanagan discloses application of the cepstrum function to multipath signal suppression, in U.S. Pat. No. 3,662,108. The complex cepstrum of a received composite signal is formed, and the inverse Fourier transform of the real part of the cepstrum is computed. This inverse transform signal is clipped about a selected center to remove some distortion components, and the remaining "mountain top" signal has a sequence of maxima at time values corresponding to arrival of the desired signal and undesired multipath replicas at the receiver. A fast Fourier transform technique for computing cepstrum functions from received signals is disclosed by Dieterich et al in U.S. Pat. No. 5,065,242, for use in removing ghost images in received television signals.
Menard discloses a multipath time delay and correlation bandwidth analyzer system in U.S. Pat. No. 3,596,182. A correlation function is formed between a signal received at the receiver and a selected reference signal generated at the receiver. If the received signal contains strong multipath contributions, displaced in time from each other, the correlation function will contain two or more correlation pulses or maxima, also displaced in time. The reference signal can be a time delayed replica of the received signal.
U.S. Pat. No. 3,605,018, issued to Coviello, discloses a nonlinear signal processing system useful for suppressing strong multipath signals in a quaternary phase, spread spectrum communications system. A received composite signal, containing the desired signal and a superimposed strong multipath signal, is passed through a circuit containing a signal envelope detector and a signal averager, arranged in parallel. The envelope detector and signal averager output signals are applied to two terminals of a differential amplifier. The diff amp output signal is an estimate of the desired signal.
A system for quantitatively measuring multipath signal distortion is disclosed by Close in U.S. Pat. No. 3,869,673. A local oscillator, mixer and amplitude limiter are applied to a received signal to provide a first signal representing frequency variation and a second signal representing amplitude variation of the received signal. A correlation signal formed from the product of these first and second signals is a quantitative measure of multipath distortion present in the received signal.
Costas discloses apparatus for minimization of distortion in signals containing multipath signals and Doppler shift effects in U.S. Pat. No. 4,349,915. Signal arrival delay, such as produced by multipath signals, is determined by examining a selected frequency contribution of spaced apart pulses that arrive at the receiver.
Gutleber, in U.S. Pat. No. 4,457,007, discloses a multipath signal interference reduction system in which a plurality of replicas of the received signal, with different time delays, amplitudes and signal widths, are sequentially subtracted from the arriving composite signal (desired signal plus multipath signals) to produce a new composite signal with markedly reduced multipath signal contribution.
An adaptive multipath distortion equalizer, disclosed by Nossen in U.S. Pat. No. 4,669,091 uses simulation of the multipath distortion, based upon the composite signals received. After this multipath distortion signal is estimated, this distortion signal is subtracted from the composite signal received to produce an estimate of the desired signal. This approach uses time delay lines, amplitude scale factors and phase adjustments to obtain the estimated multipath distortion signal.
In U.S. Pat. No. 4,672,638, Taguchi et al disclose a multipath signal canceller that uses an envelope detector to determine the shape of a received composite pulse. This shape is compared with the shape of an undistorted pulse (earliest arriving pulse) stored in the receiver. The difference between these two shapes is used to construct a distortion-cancelling signal for signals that subsequently arrive at the receiver.
Rilling discloses a multipath signal reduction system, intended to compensate for multipath signals arriving from unknown directions, in U.S. Pat. No. 4,797,950. An adaptive array of spaced apart antennas is provided, with each antenna signal being processed identically by a band pass filter, a local oscillator, a signal mixer and a tapped time delay line. The processed signals are adaptively weighted by a feedback loop and added together to provide a signal with reduced multipath contributions. Any of eight classes of signal feedback equations can be used, depending upon the choice of form of an error term used for least squares minimization of a error signal. No reference signal is required for comparison in this system. An antenna pattern null is adaptively positioned in the (unknown) direction of the arriving, unwanted multipath signals.
Schilling, in U.S. Pat. No. 5,081,643, discloses multipath receiver apparatus that compares a time delayed replica of a transmitted with the received signal and chooses a signal propagation time delay that maximizes a correlation signal formed from the first two signals.
A method for multipath signal suppression in television receivers, disclosed by David Koo in U.S. Pat. No. 5,111,298, uses transmission of a test signal that is sampled and transformed to the frequency domain. Portions of the frequency spectrum are redistributed to other frequency ranges where the spectrum amplitude is nearly zero, and the redistributed spectrum is transformed back to the time domain to produce a processed signal where echo signal contributions are suppressed. Koo discloses another method for echo signal cancellation in television receivers in U.S. Pat. No. 5,121,211, again working with the frequency domain transform of a received test signal.
Chan et al, in U.S. Pat. No. 5,127,051, disclose an modem system that senses, and adaptively adjusts itself for, the presence of multipath signals in the transmission channel. A message is decomposed into a sequence of frames, each containing a first training packet, a data packet, and a second training packet. Each training packet contains a selected sequence of binary symbols and has a length that exceeds the expected temporal length of multipath dispersion in the channel. As each three-packet frame is received, the received first and second training packets are compared with the expected first and second training packets, symbol-by-symbol, to determine and compensate for the presence of any multipath dispersion in the channel.
In U.S. Pat. No. 5,142,551, Borth et al disclose a signal weighting equalizer for adaptively filtering multipath signals received by a single antenna. The system determines and assigns confidence levels on signal accuracy to different portions of a received signal and processes these portions according to the confidence levels assigned thereto. A received signal is passed through an equalizer, including a maximum likelihood sequence estimator, at the receiver to estimate the signal originally transmitted. A synthesized transmission channel generates and modifies a synthesized "re-transmitted" signal, based upon the equalized signal, that is compared with the signal actually received at the receiver to determine confidence levels for different portions of the received signal.
A multipath detector containing a differential amplifier that senses whether the signal strength is above a first level or is below a second (lower) level is disclosed by Nakamura et al in U.S. Pat. No. 5,159,708. If the signal strength is below the second level, the system concludes that a multipath signal is present. The signal threshold for presence of a multipath signal increases approximately linearly with an average electrical field strength of a received signal.
Cai et al disclose a digital signal equalization system that senses whether multipath dispersion is present in a received, data-modulated signal, in U.S. Pat. No. 5,164,959. A correlation signal is formed between the data-modulated signal and a similar, but non-data-modulated, signal generated at the receiver. If the correlation signal has two or more peaks, the system concludes that multipath dispersion is present and estimates the transmission channel impulse response. The system then determines a set of optimum weights for the direct and multipath signals to align these signals in phase and time to estimate the signal that would have been received through a non-dispersive channel.
Belcher et al, in U.S. Pat. No. 5,203,016, disclose a signal quality-dependent recursive integrator that compares the amplitude of an emphasized signal information component with a threshold amplitude to control gain in a feedback delay loop, for analyzing arriving signals containing multipath contributions. One or more time delayed versions of the received signal is recursively summed with the originally received signal, and the feedback loop gain varies inversely with the signal-to-noise ratio of the received signal.
Minamisono, in U.S. Pat. No. 5,218,359, discloses a post-processing adaptive antenna array system with independently controllable weights and phases assigned to each antenna. Correlation functions associated with different times and directions of signal arrival are computed. The antenna weights and phases are chosen to correspond to a direction of signal arrival that produces a cumulatively high received signal level, to suppress multipath signal interference.
In U.S. Pat. No. 5,241,317, issued to Howard, multipath signal reduction apparatus for radar signal analysis for range, altitude and elevation angle is disclosed. The apparatus receives sum and difference radar return signals from a target and logarithmically amplifies these signals. A largest peak amplitude signal is selected for each of the fading sum signals and the fading difference signals, and these selected signals are used to determine a ratio that is related to target elevation angle. This approach relies on identification of peak signal values and does not directly identify a multipath signal that may be present.
Ohsawa, in U.S. Pat. No. 5,303,264, discloses an adaptive equalizer for reducing signal distortion due to multipath interference and fading in a digital system. A "signal forget factor", dependent on the signal-to-noise ratio of an incoming signal, is applied to an automatic gain controller that receives the incoming signal. The equalizer varies the waveform with forget factor included, using a recursive least squares algorithm.
Several of these patents disclose examination of a correlation signal of, or form a related comparison of, an arriving signal with a selected reference signal generated at the receiver. These approaches usually require examination of a correlation signal for the presence of two or more local maxima or peaks, which are taken to indicate the presence of multipath signals. Here, however, interest centers on mitigating the presence of multipath signals that are delayed in time by less than the width of one digital symbol time, referred to as a chip width. These multipath signals do not necessarily manifest their presence by appearance of distinct correlation peaks or local maxima, but in other, more subtle ways. What is needed is an approach allowing more accurate estimation of the arrival time of the direct path signal by means of accurate quantitative evaluation of characteristics of the non-direct or multipath signals. Preferably, this approach should allow (near) real time identification and mitigation of the multipath signals so that the arrival time of the direct signal can be estimated with greatest accuracy.